Electrochemical sensors for blood analysis in clinics, intensive care units and surgical units are well known in the art. Such sensors have been utilized for measuring various materials qualitatively and quantitatively to enhance the metabolic surveillance of patients. For instance, electrochemical sensors directed to the measurement of hydrogen, oxygen and glucose have been described in, inter alia, Kaplan, S. et. al.: Shunt Detection With Hydrogen, Ascorbate and Oxygen Electrodes. In: Intravascular Catheterization. (Eds) Zimmerman, H. A. and Thomas C. C., Springfield, Ill. (1977), U.S. Pat. Nos. 2,913,386 and 3,380,905, as well as in Clark, L. C. et. al.: Implanted Enzymatic Glucose Sensor. Diabetes Care. 5(3):174-180 (May-June, 1982), respectively. The hydrogen electrochemical sensors used in man for example can rely upon potentiometry for detecting dissolved hydrogen. To this end, such sensors measure a voltage or potential generated from the oxidation of hydrogen which is a function of the amount of hydrogen present. The oxygen and glucose electrochemical sensors on the other hand can rely upon polarography for detecting oxygen and glucose content. Polarography basically depends upon the measurement of a current at known applied voltages during the electrolysis of a solution between two or more electrodes. As to the polarographic measurement of oxygen, such a sensor is used as a cathode to detect a current generated from the reduction of oxygen by electrons which corresponds to the amount of oxygen. With respect to glucose, the sensor is generally employed as an anode to detect a current derived from the oxidation of hydrogen peroxide generated from reacting glucose with glucose oxidase in the presence of oxygen.
Unfortunately, the electrochemical sensors provided hitherto, including potentiometric and polarographic sensors, are virtually limited because of design specificity for measuring a particular chemical substrate such as hydrogen, oxygen, or glucose. Obviously, such a limitation can in some instances present undesirable drawbacks with respect to the inherent reliability of the information obtained from an electrochemical sensor notwithstanding the specific "intelligence" conferred thereto. For instance, all methods utilizing glucose oxidase to measure glucose requires the simultaneous presence of both oxygen and glucose in the material. In other words, the availability of oxygen for reaction with glucose, i.e., actual oxygen concentration in the body fluid or material, is a limiting factor for the glucose oxidation reaction. Yet, the art currently available has not heretofore accounted for such a limitation imposed by availability of oxygen on the sensors which depend on glucose oxidation for measuring glucose content in body fluids or other materials. Thus, the art is presently unable to insure that the current generated from the oxidation of glucose is in fact proportional to the glucose content in the body fluids or material being analyzed.
In one attempt to overcome the deficiency described above, U.S. Pat. No. 4,452,682 discloses an apparatus for electrochemically measuring items of blood relying upon a flow through system which comprises a blood pH/gas component sensor unit, an electrolyte component sensor unit, and/or a biochemical component sensor unit arranged in a series wherein a sampler and the sensor units are in communication with one another through a blood sample flow or channel. More particularly, the biochemical component sensor comprises an oxygen sensor electrode for detecting the dissolved oxygen concentration interconnected in a series to a glucose sensor electrode for measuring the glucose content.
In another attempt directed to the deficiencies aforementioned, U.S. Pat. No. 4,431,004 discloses a method and apparatus for more accurate measurements of glucose content in body fluids by sensing the absolute level of oxygen concentration in the fluid and correcting the output differential measurement indicative of the glucose content in the fluid according to the absolute level of oxygen. To accomplish the above, U.S. Pat. No. 4,431,004 employs an unaltered oxygen sensor interconnected to an altered oxygen sensor formed into a glucose sensor and positioned between the unaltered oxygen sensor and the body fluid to be analyzed for measuring the level of oxygen and glucose which constitutes a more accurate reading for the glucose content in the body fluid being analyzed.
Consequently, all of the electrochemical sensors provided hitherto invariably necessarily require a series or combination of sensors to measure a substrate and a chemical entity such as oxygen to assure a more accurate and reliable reading of a particular substrate being analyzed. Such a requirement for a series or combination of sensors contributes to the overall size of the electrochemical sensor adversely affecting the feasibility and undesirability of implantability of the sensors for in vivo detection. Thus, there are needs for electrochemical sensors having the capability of measuring with specificity a substrate and a chemical entity such as oxygen to help better determine the reliability of the substrate being detected.